This invention relates to control apparatus for controlling the operation of an air compressor, in a system driven by the compressor, to avoid stall or surge and to improve efficiency. Surge, surging or stall in an air compressor-driven system is an unstable condition that may occur when, for example, the flow rate of the air through the compressor drops below a given minimum. It is an oscillation or the onset of oscillation in which the flow rate increases and decreases alternately, and the air flow may even reverse its direction through the compressor. Surging causes excessive vibrations and noise, and may cause permanent damage to the air compressor.
In order to control surge, it is first necessary to understand what causes surge. FIG. 1 in the drawings is a plot of pressure versus flow (Q) for the source (air compressor) and the load (pipes and tank system). It is well known that if the slope of the source is less than the slope of the load, the system will be stable. Thus, it would be well to design a compressor whose slope on the pressure vs. flow plot is always negative and then, because a passive load always has its slope greater than zero, the slope of the source will always be less than the slope of the load. FIG. 2 shows a plot of pressure versus flow for a compressor at various speeds (N) of the compressor. The line A intersects the pressure-flow curves for various speeds. If the flow in the system is measured and the speed is then changed to always remain on line A, the compressor characteristic will always have a slope which is negative. Only flow need be measured because the P versus Q curves for various speeds are known and it is only necessary to set the speed for the flow desired. This is shown in FIG. 3. As the load requires a different operating point, the value of "b" shown in FIG. 3 will be matched to the load.
It has thus been described how the compressor curves can be made to always have a negative slope. It is, however, only necessary to have an effective negative slope dynamically, and statically the old characteristics will remain. FIG. 4 shows the compressor characteristic for various speeds. Line S is known as the surge line and connects all of the maxima from the compressor characteristic for the different speeds. The "surge region" is that area to the left of the surge line S and the "non-surge region" is the area to the right. Point O is an operating point within the surge region. By dynamically controlling the speed of the compressor, a negative slope of the pressure versus flow of the compressor about the operating point O can be maintained. Note that the system can be operated stably in what is normally known as the surge region without any accessory system loading means. It will be shown that such a negative slope is always maintained in the system of the present invention and thus the system can actually operate within the surge region and stability will always occur and no surging will take place.
The present invention may be applied to any system having a variable load which is driven by an air compressor, such as a turbine, pump, fan, jackhammer, electric generator, jet engine, etc. It is particularly useful when incorporated in an electric power generating system having a turbine-driven generator and a turbine-driven air compressor and will be described in that environment.
Unfortunately, all of the previous attempts to avoid surge have substantially lessened the efficiency of the air compressor. For example, in one prior control arrangement, when the possibility of surging exists guide vanes at the compressor inlet (usually called prerotation vanes or PRV) are adjusted to throttle the input flow to introduce drag or resistance in an amount sufficient to prevent surging. This changes the load characteristic of the system such that over the desired operating range the net system impedance has a positive resistance. In another control scheme, when a surging condition is approached, gas is bypassed from the compressor discharge to the inlet or is merely passed through a blow-off line. In effect, an additional load is placed on the system such that the impedance of the total system exhibits a net positive resistive component. These prior solutions to the surge problem employ passive and inefficient mechanical mechanisms that dissipate considerable energy. The surge prevention arrangement of the present invention, on the other hand, constitutes a significant improvement over the systems developed heretofore since essentially no energy is needed to keep the air compressor-driven system out of surge. Moreover, additional economies are realized in that the construction of the air compressor may be simplified considerably from that which is usually required.